Increased fungal dominance in N2O emission hotspots along a natural pH gradient in organic forest soil

Drained organic forest soils represent a hotspot for nitrous oxide (N2O) emissions, which are directly related to soil fertility, with generally higher emissions from N-rich soils. Highest N2O emissions have been observed in organic forest soils with low pH. The mechanisms for these high emissions are not fully understood. Therefore, the present study was conducted to gain a deeper insight into the underlying mechanisms that drive high N2O emissions from acid soils. Specifically, we investigated the microbial community structure, by phospholipid fatty acid analysis, along a natural pH gradient in an organic forest soil combined with measurements of physico-chemical soil properties. These were then statistically related to site-specific estimates of annual N2O emissions along the same natural pH gradient. Our results indicate that acidic locations with high N2O emissions had a microbial community with an increased fungal dominance. This finding points to the importance of fungi for N2O emissions from acid soils. This may either be directly via fungal N2O production or indirectly via the effect of fungi on the N2O production by other microorganisms (nitrifiers and denitrifiers). The latter may be due to fungal mediated N mineralization, providing substrate for N2O production, or by creating favourable conditions for the bacterial denitrifier community. Therefore, we conclude that enhanced N2O emission from acid forest soil is related, in addition to the known inhibitory effect of low pH on bacterial N2O reduction, to a soil microbial community with increased fungal dominance. Further studies are needed to reveal the exact mechanisms.